Charged-particle device having magnetic focus means

ABSTRACT

A charged-particle device wherein a magnetostatic field for focus of charged particles is generated by a combination of an electromagnet subject to line voltage variations and a second magnet not subject to line voltage variations, each component contributing an equal amount of flux. Current for the electromagnet component, and anode voltage for the beam, are derived from the same source of power. Under these conditions, the system becomes &#39;&#39;&#39;&#39;self-regulating,&#39;&#39;&#39;&#39; in that size and position of an electron image becomes immune to wide variations of line voltage as well as voltage transients.

United States Patent Schlesinger 51 Aug. 1, 1972 CHARGED-PARTICLE DEVICE HAVING MAGNETIC FOCUS MEANS Kurt Schlesinger, Fayetteville, N.Y.

Assignee: General Electric Company Filed: Feb. 26, 1970 Appl. No.: 14,334

lnventor:

US. Cl. ..315/31, 313/84, 313/154 Int. Cl ..IIOlj 29/52 Field of Search .I'..328/228; 315/299, 343, 30,

References Cited UNITED STATES PATENTS 12/ l 947 Bradley ..313/84 4/1952 Van Cjilder ..313/84 l/1956 Flory ..313/154 X 7/1965 Nakayama et ai. ..313/84 X dNODE HIGH VOLTAGE RECTIFIER POWER SOURCE ELECTRO- MAGNETIC FOCUS COIL RECTIFIER Primary Examiner-Benjamin R. Padgett Assistant Examiner-J. M. Potenza AttorneyNathan J. Cornfeld, John P. Taylor, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman [57] ABSTRACT A charged-particle device wherein a magnetostatic field for focus of charged particles is generated by a combination of an electromagnet subject to line voltage variations and a second magnet not subject to line voltage variations, each component contributing an equal amount of flux. Current for the electromagnet component, and anode voltage for the beam, are derived from the same source of power. Under these conditions, the system becomes self-regulating," in that size and position of an electron image becomes immune to wide variations of line voltage as well as voltage transients.

11 Claims, 3 Drawing Figures P'A'TENTEDAuc 1 I972 3.681.651

sum 1 or 2 I9 I7 I5 13 II ANODE HIGH VOLTAGE RECTIFIER P0 WER SOURCE ELECTRO- MAGNETIC FOCUS COIL RECTIFIER INVENTOR: KURT SCHLESINGER,

BY HIS ATT RN PATENTED H973 3.681.651

sum 2 0F 2 FIG.3.

(9 I7 15 I3 II ANODE man VOLTA as RECTIFIER A, POWER saunas 8 -10 ELECTRO- MAGNETIC 4 FOCUS COIL 3o RECTIFIER CHARGED-PARTICLE DEVICE HAVING MAGNETIC FOCUS MEANS BACKGROUND OF THE INVENTION This invention relates to the magnetic focusing of charged particles in charged-particle devices. In one aspect the invention relates to the magnetic focus of electron beams in image tube devices.

Magnetic focusing of charged-particle devices such as an image tube wherein a common power source is used to supply current to the magnetic focusing coil and voltage to the anode has presented problems with regard to defocusing and image deformation caused by line voltage fluctuations. Solutions previously proposed include the utilization of DC voltage, or current regulation equipment, or the use of non-linear networks in connection with the coil to effect compensating adjustments in the magnetic field in proportion to the change in screen voltage. These solutions have presented problems with regard to weight, complexity of circuits and attendant expense.

Gethmann U.S. Pat. No. 2,483,133, assigned to the assignee of this invention proposed a solution wherein a permanent magnet and an electromagnet were both used to focus the beam. The permanent magnet field was made such that it focused the electron beam at an accelerating potential of half the normal beam operating potential. While this approach reduced the amount of defocusing, the problem was not solved because the change in the accelerating potential with respect to the change in the magnetic field of the electromagnetic was not linear.

It is therefore an object of the invention to provide an improved system for the magnetic focusing of charged particles in a charged-particle device which provides protection or conservation of the focus despite the voltage variations. Further objects of the invention will be apparent from the description of the preferred embodiments.

SUMMARY OF THE INVENTION In accordance with the invention, a charged-particle device having a source of charged particles and beam forming electrode means spaced from the source to provide a beam of charged particles is focused by a magnetic focusing system wherein the magnetic field is generated by a combination of two independent field generator means, the first means being subject to variations in line potential and the second means being not subject to variations in line potential, each contributing substantially identical field patterns of equal strength in the volume occupied by the beam whereby the focus of the beam of charged particles is practically immune to line voltage variations when the voltage on the beamforming electrode means and the current to the first magnetic focus means are obtained from a common power source.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal cross-sectional representation of an image tube using the focus system of the invention.

FIG. 2 is a cross-sectional end view of the tube in FIG. 1 taken along 2-2.

FIG. 3 is a longitudinal cross-sectional representation of another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. I an image tube generally indicated at 2 comprises an evacuated cylinder 4 having a photosensitive electrOn emitting member 6 adjacent a first end wall of cylinder 4 and a target member 8 on the opposite end wall of cylinder 4. Adjacent target member 8 is a screen electrode 10 which is electrically connected to the positive terminal of the output of a high voltage rectifier 20. The negative terminal of rectifier 20 is attached to electron emitting member 6. Cylindrical electrodes l2, l4, l6, and 18 are mounted in cylinder 4 between emitting member 6 and screen electrode 10. The potential on these electrodes is also derived from rectifier 20 by a bleeder system including resistors 11, l3, l5, l7, and 19 as shown in FIG. 1. It should be noted therefore, that all of the electrodes used in forming the beam are energized from a common power source.

Surrounding cylinder 4 is an electromagnetic focus coil 30 energized by focus coil rectifier 32. Coil rectifier 32 and high voltage rectifier 20 are in turn energized by one common power source 40. Variable resistor 34 in series with focus coil 30 acts as a fine focus control to adjust the focus.

In accordance with a preferred embodiment of the invention, a coaxial system of permanent bar magnets 50 surrounds coil 30. As more clearly shown in FIG. 2, the bar magnets are symmetrically spaced coaxially around the cylinder. Although 8 magnets are illustrated, the actual number necessary to supply the desired field may vary depending upon the strength of the magnets, the physical dimensions of the cylinder and focus coil, and the amount of magnetic flux desired as well as the area of a plane normal to the tube axis in which field uniformity is desired. Although bar magnets are illustrated, other geometrical shapes such as, for example, toroidal rings could also be used provided, of course, that the orientation of the polarity was correct. The use of ring magnets, for example, would allow some control over the axial distribution of the field in cylinder 4 which could be useful under certain conditions.

It will be noted that the permanent bar magnets 50 in FIG. I extend at each end beyond cylinder 4 and coil 30 surrounding cylinder 4. These extensions are provided because, as is well known to those skilled in the art, the usable portion of the permanent magnetic field having a field distribution similar to a long coil is actually less than the total length of bar magnet 50. The exact length of bar magnet 50 is therefore chosen to provide an effective permanent magnetic field distribution comparable to the electromagnetic field from coil 30 in the volume occupied by the beam. Stated another way, the total length of bar magnets 50 and their distribution around cylinder 4 are chosen to provide a magnetic field of substantially identical field pattern and equal strength to that provided by coil 30 within cylinder 4.

In accordance with the invention, permanent magnets 50 and focus coil 30 are designed to each contribute and approximately equal amount of effective magnetic flux. Under these conditions, the focus system becomes self regulating with respect to line variations. Since all beam-forming voltages and focus coil current are supplied by a common line voltage,

variations in this line voltage, therefore, also results in no change of particle trajectory and therefore no change of focus. It has been demonstrated that variations in line voltage can now be tolerated over a wide range, for example, as much as 2030 percent, without focus degradation.

These experimental observations appear to be explainable mathematically, although it is to be understood that I do not wish to be bound by any particular theory of operatiOn. The relationship of the magnetic field strength to the voltage and the length of the field can be expressed as:

BL constant J where: B field strength V= ultor voltage L length of field Rewriting Equation 1 in differential form:

Equation 1 Equation 2 Since there is to be no change in image position, A L =0. It is then seen that any accidental change of ultor voltage (A V) has to be countered by a similar but smaller change of the magnetic field (A B) by rewriting Equation 2 with AL=O:

" Equation B B Bp Equation then the change-differential of the permanent magnetelectromagnet combination can be expressed as:

A B,/B, A 8 /8,. B,,

Equation 5 If the two field contributions, B and B,,, are chosen to be equal under equilibrium conditions, i.e. where AB 0, the combined total field changes only half as much as the electromagnetic component by itself and equation 5 can be rewritten as:

A B lB, AB /B Equation 6 Thus Equation 6 shows that the total field B, can be made to change only one-half as fast as the current in the electromagnet by selecting a permanent magnet of field strength approximately equal to the electromagnetic field strength under equilibrium conditions.

Combining Equations 3 and 6:

The above equations accordingly appear to confirm the experimental results obtained using the magnetic focus system of the invention.

My invention may also be practiced in accordance with a second embodiment (as .shown in H0. 3) wherein the permanent magnets are replaced by a second electromagnet 60. The coil 60 of the second electromagnet is wound about cylinder 4 coaxially with coil 30 and in the same direction. The winding of the second coil may be wound bifilar to coil 30 and polarized in an aiding rather than bucking direction. This second coil is, however, not electrically connected to coil 30, but rather is energized by a power source which is not subject to line voltage variations such as, for example, a battery. In accordance with the invention, the ampere-turn density per inch on the second coil is preselected to be such that the second coil provides a field equal to that supplied by coil 30 at equilibrium. It can be seen then that variations in line voltage or voltage transients will then only affect coil 30 and therefore only one-half of the total magnetic field will vary. Thus, the explanations offered for the operation of the preferred embodiment should also apply when one-half of the magnetic field strength is provided by an electromagnet energized by a power source not subject to line voltage variations.

The invention, it should be noted, therefore provides a charged particle device with an improved focus system, with respect to voltage variations, over a single coil all electromagnetic focus, an all permanent magnet focus, and even mixed fields where the ratio varies significantly from half of the total field strength for each field since voltage variations will disturb the focus in any of these prior art systems as can be seen in the equations above. The invention, by providing a mixed magnetic field, rather than all permanent magnet, also provides for fine focus control by allowing adjustment of the electromagnetic field within limits. This adjustment, it should be noted, can be provided in either coil when two coils are used as in the second embodiment described.

Thus, the invention provides a magnetic focus system for the electron beam of an image tube device wherein the focus is relatively immune to line voltage fluctuations of as much as 20-30 percent when the screen voltage and focus coil current for one-half of the total magnetic field strength are obtained from a common power source. The focus is maintained without the use of expensive and heavy voltage regulators (with their inherent inefficiency since excess voltage must be generated and wasted) or complicated electronic circuitry. The invention offers the power saving of (4:1) by the use of permanent magnets for one-half of the field strength, or in the second embodiment, the compactness of electromagnets together with, in either embodiment, the focus stability of permanent magnetic focusing and the tunability of focus of electromagnetic focusing.

While the novel focusing system has been illustrated in a particular image tube it is to be understood that the invention can be used in any type of charged-particle device wherein the maintenance of focus of charged particles independent of line voltage fluctuations is desired. The invention may be useful in the focusing of either negatively or positively charged particles such as, for example, in a mass spectrometer or in color tube focus and convergence circuitry. The use of the term charge-particle device is therefore intended to inelude devices where beams of either negatively charged particles or positively charged particles are focused. The invention is therefore to be limited only by the scope of the appended claims:

What I claim as new and desire to secure by patent of the United States is:

1. A charged-particle device having:

a. a source of charged particles;

b. a charged particle attracting electrode spaced apart from said source to create a beam of charged particles therebetween;

c. magnetic focus means including;

1. first magnetic focus means substantially immune to line voltage variations, and

2. second 7 magnetic focus means comprising an electromagnet said first and second magnetic focus means being of substantially equal field strength,

d. a common power source supplying voltage to said electrode and current to said second magnetic focus means, whereby the focus of said beam of charged particles is relatively immune to line voltage variations.

2. The electro-optical device of claim 1 wherein said first magnetic focus means comprises permanent magnet means having an effective field strength substantially equal to the field strength of said electromagnet.

3. The electro-optical system of claim 2 wherein said permanent magnet means comprise bar magnets symmetrically spaced about said electromagnet parallel to the axis of said beam and extending beyond said electromagnet toward said source and said electrode a sufficient distance to provide an effective magnetic field strength equal to that of the electromagnet.

4. The electro-optical system of claim 2 wherein said permanent magnet means comprise ring magnets.

' 5. The electro-optical device of claim 1 wherein said first magnetic focus means comprises an electromagnet.

6. The electro-optical device of claim 5 wherein said electromagnet of said first magnetic focus means is energized by a direct current power source.

7. The electro-optical device of claim 6 wherein said direct current power source comprises a battery.

8. A charged-particle device having:

a. a source of charged particles;

b. a charged particle attracting electrode means spaced apart from said source to create a beam of charged particles therebetween;

c. magnetic focus means therebetween including 1. first magnetic focus means substantially immune to line voltage variations, and 2. second magnetic focus means comprising an electromagnet; said first and second magnetic focus means having substantially equal magnetic field distributions to provide substantially identical field patterns of equal strength; and

d. a common power source for said electrode means and said second magnetic focus means whereby the focus of said beam of charged particles is substantially immune to voltage variations in said common power source.

9. The device of claim 8 wherein said first magnetic focus mean com sa ermanent ma net.

10. A e if-regmfing nagnetic focu s system for a charged particle device having a power source subject to line voltage variations and having therein a source of charged particles and beam-forming electrode means spaced therefrom and connected to said power source comprising:

Two magnetic field generating means between said sOurce and said electrode means to focus said beam of charged particles each contributing substantially identical field patterns of equal strength in the volume occupied by said beam.

. one of said magnetic field generating means comprising an electromagnet connected to said power source, and 2. the other of said magnetic field generating means being substantially immune to line voltage variations;

whereby line voltage variations changing the voltage on said electrode means are compensated by the adjustments in the magnetic field of said electromagnet to preserve the focus of the beam.

11. The system of claim 10 wherein said other magnetic field generating means comprises a permanent magnet. 

1. A charged-particle device having: a. a source of charged particles; b. a charged particle attracting electrode spaced apart from said source to create a beam of charged particles therebetween; c. magnetic focus means including;
 1. first magnetic focus means substantially immune to line voltage variations, and
 2. second magnetic focus means comprising an electromagnet said first and second magnetic focus means being of substantially equal field strength, d. a common power source supplying voltage to said electrode and current to said second magnetic focus means, whereby the focus of said beam of charged particles is relatively immune to line voltage variations.
 2. second magnetic focus means comprising an electromagnet said first and second magnetic focus means being of substantially equal field strength, d. a common power source supplying voltage to said electrode and current to said second magnetic focus means, whereby the focus of said beam of charged particles is relatively immune to line voltage variations.
 2. The electro-optical device of claim 1 wherein said first magnetic focus means comprises permanent magnet means having an effective field strength substantially equal to the field strength of said electromagnet.
 2. second magnetic focus means comprising an electromagnet; said first and second magnetic focus means having substantially equal magnetic field distributions to provide substantially identical field patterns of equal strength; and d. a common power source for said electrode means and said second magnetic focus means whereby the focus of said beam of charged particles is substantially immune to voltage variations in said common power source.
 2. the other of said magnetic field generating means being substantially immune to line voltage variations; whereby line voltage variations changing the voltage on said electrode means are compensated by the adjustments in the magnetic field of said electromagnet to preserve the focus of the beam.
 3. The electro-optical system of claim 2 wherein said permanent magnet means comprise bar magnets symmetrically spaced about said electromagnet parallel to the axis of said beam and extending beyond said electromagnet toward said source and said electrode a sufficient distance to provide an effective magnetic field strength equal to that of the electromagnet.
 4. The electro-optical system of claim 2 wherein said permanent magnet means comprise ring magnets.
 5. The electro-optical device of claim 1 wherein said first magnetic focus means comprises an electromagnet.
 6. The electro-optical device of claim 5 wherein said electromagnet Of said first magnetic focus means is energized by a direct current power source.
 7. The electro-optical device of claim 6 wherein said direct current power source comprises a battery.
 8. A charged-particle device having: a. a source of charged particles; b. a charged particle attracting electrode means spaced apart from said source to create a beam of charged particles therebetween; c. magnetic focus means therebetween including
 9. The device of claim 8 wherein said first magnetic focus means comprises a permanent magnet.
 10. A self-regulating magnetic focus system for a charged particle device having a power source subject to line voltage variations and having therein a source of charged particles and beam-forming electrode means spaced therefrom and connected to said power source comprising: Two magnetic field generating means between said sOurce and said electrode means to focus said beam of charged particles each contributing substantially identical field patterns of equal strength in the volume occupied by said beam.
 11. The system of claim 10 wherein said other magnetic field generating means comprises a permanent magnet. 